US to Asia will be literally a backup routing, since that the best routes between Japan and western US coast are already continuously constructed and maintained. EU and Japan would be godsend, the best direct link would be through China, which is politically and practically (even with the best of diplomatic relations, terrestrial cable linking is much more complicated due to mountains, rivers and other obstacles) impossible. Although there's an EU-Hong Kong terrestrial link operated by RETN, it's a relatively low-bandwidth link compared to the aggregate of all Japan-Singapore-Marseille links, so if this a high-bandwidth link it'll be a boon.
Unfortunately probably none, unless you live pretty far north. We're pretty close to the speed of light limit* when it comes to latency between the US and Asia, which makes sense as a straight line is over the Pacific anyway. (e.g., I get ~1.1x the theoretical min latency pinging Tokyo from my Fiber connection in the SF Bay Area)
Where it seems like we're pretty far from the theoretical min is connections within the continental US. Latency is pretty bad (e.g. I get like 1.5x - 4x the theoretical min latency from my Fiber connection in the SF Bay Area, depending on endpoint). I assume part of this is indirect connections (you don't have a direct fiber connection between every pair of cities, because that would be dumb) and some of it is routing overhead (a connection to Asia goes a long distance, but often has way fewer hops).
Note that this is theoretical min latency based on the speed of light through fiber*, which is a bit higher than (about 5/3x) the speed of light through air. New fiber optic tech might help this at some point.
Your intuition is correct, undersea cables are close to direct connections while overland routes are interconnects so you'll have tons of hops. What we have for overland gets the job done so it's hard to justify the massive cost of installation. Plus there's likely competing business interests at play.
